A question about voltage levels

Newbuggyboy

In the 91 marathon. It have voltage everywhere all the time. Even with the ignition switch off. The solenoid has pack voltage on one side large terminal. And -3 volt on the other large terminal. That lower voltage is at the controller B+. The fnr cables have somewhere in the 27V range. With the fnr in neutral when I press the accelerator the cart doesn't move. Forward and reverse work properly when selected and the pedal is pushed. The ignition switch I shows zero with the key off.

Is this normal?

I have disconnected the pack + into the solenoid after driving the cart because I don't understand this. It takes time and all those voltages return to zero with the pack disconnected.

Edit note: wiring is straight from the battery diagram and the solid state diagram. (I believe)

BGW

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JohnnieB

There are no perfect insulators and digital voltmeters have a very high input impedance, so it doesn't take much of a current path to get a voltage reading where it hardly exists. This wasn't an issue with analog voltmeters because they were typically about 20kΩ/V, but modern DVMs are 1MΩ (or greater), so if the undesired current path is 5MΩ, they will read 1/5 of the voltage potential between the two points the test leads are touching. Technically, the analog voltmeter will read a voltage also, but the ratio between the meters input impedance and the undesired current path's resistance is so high the needle barely moves.

If you connect the negative test lead to BR- (B-) at the battery pack on the attached schematic, all voltage readings will be positive.

The reason the voltages drop off slowly when the battery pack is disconnected is due to there being a dozen or so filter capacitors across the dozen or so MOSFETs in the controller. For safety, the filter capacitors have a bleeder network, but it take a few minutes for the charge on the filter capacitors to be bled off. (Curtis 1204 block diagram attached)

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FWIW: The phenomenon that causes DVMs to read "phantom" voltages is called leakage current (electricity leaking through insulators) is something I dealt with for 35 years due to medical devices have codified electrical leakage standards imposed on them by various local, state and federal agencies. The standards are for AC, but the phantom voltage appear with DC also.

I had a dual banana plug with an AMMI test load attached that I plug into my DVM and then pug the test leads into the banana plug jacks when I use one of my DVMs to do a quick check on some electrical item. (schematic attached) For DC, just place a 1,000Ω resistor between the test leads and the phantom voltages will all but disappear. If they don't there is a problem to be found and fixed.

Attached Images

	Pot Box cart schematic.jpg (110.7 KB, 0 views)
	Curtis 1204 block diagram.JPG (55.5 KB, 0 views)
	AMMI Test Load.jpg (27.7 KB, 0 views)

Newbuggyboy

I read your response and say ok. But the strange thought I have is why would the two sides of the solenoid show the current pack voltage and the other side show -3 volts less. Any reason? And those are the same numbers when I turn the ignition switch on.

JohnnieB

When they solenoid is activated (contacts closed) both of the large terminals ought to have the same voltage on then. If not the solenoid's contacts are bad and it needs to be replaced.

When the solenoid is not activated (and the is a precharge resistor between the large terminal) the battery side terminal will read a few volts higher than the controller side large terminal. This is due to a small current passing through the precharge resistor keeping the filter capacitors, which have a bleeder network, charged.

If your precharge resistor is 250Ω and there is a 3V drop, the bleeder network is drawing 12mA.

Newbuggyboy

Can I use an RXV 36V solenoid? I tested it with the 36 volt + / -. it clicks.

Are there any differences? What?

JohnnieB

I don't know how many amps the contacts are rated for, but it'll probably work.

ONEDAY

I didn't know the RXV had a 36 volt solenoid. I thought they were all 48 volts

kernal

Could be either. Depends on the year. But all RXV's are 48v.

nickdalzell1

I deal with a few folks who didn't know a TXT gas car has a 14 volt solenoid.

Newbuggyboy

Here is the situation. The original solenoid had pack voltage on the small terminals and one large terminal (the one in the back connected to pack + .) The other large terminal (the one in the front which goes to controller ) had pack -3v. These were all the time key on, key off. (Daniel son wax on, wax off.) I never heard any click from it. The cart runs fine.

I tested the rxv solenoid before the swap. Clicked and tested good. The only difference is the is zero voltage at both small terminals all the time. Even with "wax on, wax off." And I hear no clicking. The cart runs fine.

I have never tested either one while pressing the accelerator. Because I don't have a helper, or the know how. "Crane kick no can defend."